Origin of Species Variorum

yet, at .. remote geological periods, the earth may have been almost as well peopled with .. species of many genera, families, orders, and classes, as at the present day.

On the Degree to which Organisation tends to advance.

Natural Selection exclusively acts by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions of life to which each creature is exposed at each successive period of life. The ultimate result is that each creature .. tends to become more and more improved in relation to their conditions. .. .. This improvement ... inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each others satisfaction what is meant by an advance in organisation. Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs pass through in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva. Von Baers standard seems the most widely applicable and the best, namely, the amount of differentiation of the different parts of the same organic being, in the adult state as I should be inclined to add, and their specialisation for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure this subject .. is if we look, for instance, to fish, amongst which some naturalists rank those as highest